This invention relates to recording/reproduction of encoded data (compressed data) obtained on bit compression of digital audio signals, a recording medium on which the encoded data are recorded, and a transmission system for the encoded data. More particularly, it relates a method and apparatus for encoding (compressing) digital data in which the frequency-based size of mini-blocks for bit allocation for floating and/or encoding for information compression, sub-divided in time and frequency, is changed based upon changes in the input signal on the frequency axis, and a recording medium having encoded (compressed) signals recorded thereon.
The present Assignee has already proposed in e.g., the U.S. Pat. No. 5,243,588 a technique of bit-compressing input digital audio data for burst-like recording the resulting data in terms of a pre-set amount of data as a recording unit.
This technique records/reproduces adaptive differential PCM (ADPCM) as prescribed in audio data formats of the so-called CD-Interactive (CD-I) or CD-ROM XA, using a magneto-optical disc as a recording medium. The PCM audio data are recorded in a burst-like fashion on the magneto-optical disc, in terms of e.g., 32 sectors of the ADPCM data and a few linking sectors for interleaving as a recording unit.
Several modes may be selected for the ADPCM audio for the recording/reproducing apparatus employing the magneto-optical disc. For example, based upon the playback time for the usual CD, a level A with a compression factor of 2 and a sampling frequency of 37.8 kHz, a level B with a compression factor of 4 and a sampling frequency of 37.8 kHz and a level C with a compression factor of 8 and a sampling frequency of 8.9 kHz, are prescribed. For example, with the level B, digital audio data are compressed to approximately 1/4 such that the play time of the disc recorded with the mode of the level B is four times that of the disc of the standard CD format (CD-DA format). Since the recording/play time substantially as long as that of the standard 12 cm disc may be achieved, the apparatus may be reduced in size.
However, since the rotational velocity of the disc is the same as that of the standard CD, compressed data corresponding to the four-fold play time can be obtained per pre-set time for the level B. Thus the same compressed data is read in quadruplicates with time units e.g., of sectors or clusters. Only one of the quadruplicate readouts of the compressed data is passed to audio reproduction. Specifically, when scanning or tracking a spirally extending recording track, the same track is repeatedly tracked four times by effecting track jump of reverting to the original track position per each rotation in executing the playback operation. This means that it suffices to obtain normal compressed data for only one of the quadruplicate readout operations. Thus the above arrangement is desirably applied to a portable small-sized equipment which is strong against errors such as disturbances.
For compressing digital data by utilizing the above technique, it is effective to utilize psychoacoustic characteristics of the human hearing sense, that is the so-called masking effects or minimum audibility characteristics, for achieving efficient compression. For exploiting these psychoacoustic characteristics, it is necessary to analyze the input signal according to its respective frequency areas. To this end, orthogonal transform and/or a frequency-dividing filter is employed. The frequency-domain spectral data resulting from the orthogonal transform or data divided into at least two areas by the frequency-dividing filter may be handled as frequency-domain data. Thus the above technique may be utilized for changing the signal characteristics on the frequency domain, power representation on the frequency basis or representation of so-called spectrum analyzer without the necessity of adding a system or devices.
In the present specification, spectral data are frequency-domain data obtained on orthogonal transform by MDCT. However, the MDCT is given only for illustration and is not intended as limiting the invention. Thus the data obtained on fast Fourier transform FFT, discrete Fourier transform (DFT) or discrete cosine transform (DCT), and spectral data divided in frequency by a band-pass filter, such as a quadrature mirror filter (QMF), are collectively termed spectral data.
With the above-described technique of producing frequency-domain data using the above frequency-dividing filter, it is a usual practice to divide signals into frequency bands and thin Out data by decimation for preventing the number of data from being increased. Although the data decimation produces aliasing noise due to non-ideal characteristics of the frequency-dividing filter, this aliasing noise usually is not inconvenient if a synthesis filter for decoding the encoded data is selected for satisfying the condition of canceling the aliasing noise.
However, if the frequency-domain data are directly employed by operating on frequency-domain characteristics as described above, the condition of canceling the aliasing noise is not met. The result is that the decoded music data contains the aliasing noise thus raising problems in connection with psychoacoustic characteristics of the human hearing system, that is deterioration in the sound quality. Since the aliasing noise is basically produced by incompleteness in division or defect in the synthesis filter, the number of orders of the filter may be increased for approaching the filter characteristics to those of a an ideal filter for eliminating the effect of the aliasing noise. However, the amount of the arithmetic operations in this case is voluminous and is not desirable from the viewpoint of practical circuit scale.